Shan-Tao Zhang

Giant strain in lead-free piezoceramics Bi0.5Na0.5TiO3–BaTiO3–K0.5Na0.5NbO3 system

Piezoelectric actuators convert electrical into mechanical energy and are implemented for many large-scale applications such as piezoinjectors and ink jet printers. The performance of these devices is governed by the electric-field-induced strain. Here, the authors describe the development of a class of lead-free (0.94−x)Bi0.5Na0.5TiO3–0.06BaTiO3–xK0.5Na0.5NbO3 ceramics. These can deliver a giant strain (0.45%) under both unipolar and bipolar field loadings, which is even higher than the strain obtained with established ferroelectric Pb(Zr,Ti)O3 ceramics and is comparable to strains obtained in Pb-based antiferroelectrics.Piezoelectric actuators convert electrical into mechanical energy and are implemented for many large-scale applications such as piezoinjectors and ink jet printers. The performance of these devices is governed by the electric-field-induced strain. Here, the authors describe the development of a class of lead-free (0.94-x)Bi0.5Na0.5TiO3-0.06BaTiO(3)-xK(0.5)Na(0.5)NbO(3) ceramics. These can deliver a giant strain (0.45%) under both unipolar and bipolar field loadings, which is even higher than the strain obtained with established ferroelectric Pb(Zr,Ti)O-3 ceramics and is comparable to strains obtained in Pb-based antiferroelectrics.

Substitution-induced phase transition and enhanced multiferroic properties of Bi1−xLaxFeO3 ceramics

Single-phase, insulating Bi1−xLaxFeO3 (BLFOx, x=0.05, 0.10, 0.15, 0.20, 0.30, and 0.40) ceramics were prepared. An obvious phase transition from rhombohedral to orthorhombic phase was observed near x=0.30. It is found that the phase transition destructs the spin cycloid of BiFeO3 (BFO), and therefore, releases the locked magnetization and enhances magnetoelectric interaction. As a result, improved multiferroic properties of the BLFO0.30 ceramics with remnant polarization and magnetization (2Pr and 2Mr) of 22.4μC∕cm2 and 0.041emu∕g, respectively, were established.

Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3–BaTiO3–K0.5Na0.5NbO3. I. Structure and room temperature properties

Lead-free piezoelectric ceramics, 1� xyBi 0.5 Na 0.5 TiO 3 -xBaTiO 3 -yK 0.5 Na 0.5 NbO 3 0.05x 0.07 and 0.01y 0.03, have been synthesized by a conventional solid state sintering method. The room temperature ferroelectric and piezoelectric properties of these ceramics were studied. Based on the measured properties, the ceramics were categorized into two groups: group I compositions having dominant ferroelectric order and group II compositions displaying mixed ferroelectric and antiferroelectric properties at room temperature. A composition from group II near the boundary between these two groups exhibited a strain as large as 0.45% at an electric field of 8k V/ mm. Polarization in this composition was not stable in that the piezoelectric coefficient d33 at zero electric field was only about 30 pm/ V. The converse piezoelectric response becomes weaker when the composition deviated from the boundary between the groups toward either the ferroelectric or antiferroelectric compositions. These results were rationalized based on a field induced antiferroelectric-ferroelectric phase transition.

Morphotropic phase boundary in (1−x)Bi0.5Na0.5TiO3–xK0.5Na0.5NbO3 lead-free piezoceramics

The electromechanical behavior of (1−x)Bi0.5Na0.5TiO3–xK0.5Na0.5NbO3 (BNT-KNN) lead free piezoelectric ceramics is investigated for 0⩽x⩽0.12 to gain insight into the antiferroelectric-ferroelectric (AFE-FE) phase transition on the basis of the giant strain recently observed in BNT-based systems. At x≈0.07, a morphotropic phase boundary (MPB) between a rhombohedral FE phase and a tetragonal AFE phase is found. While the piezoelectric coefficient is largest at this MPB, the total strain further increases with increasing KNN content, indicating the field-induced AFE-FE transition as the main reason for the large strain.

Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3–BaTiO3–K0.5Na0.5NbO3. II. Temperature dependent properties

The temperature dependence of the dielectric and ferroelectric properties of lead-free piezoceramics of the composition (1−x−y)Bi0.5Na0.5TiO3–xBaTiO3–yK0.5Na0.5NbO3 (0.05⩽x⩽0.07, 0.01⩽y⩽0.03) was investigated. Measurements of the polarization and strain hystereses indicate a transition to predominantly antiferroelectric order when heating from room temperature to 150°C, while for 150<T<200°C both remnant polarization and coercive field increase. Frequency-dependent susceptibility measurements show that the transition is relaxorlike. For some samples, the transition temperature Td is high enough to allow mostly ferroelectric ordering at room temperature. These samples show a drastic increase of the usable strain under an external electric field just after the transition into the antiferroelectric state at high temperatures. For the other samples, Td is so low that they display significant antiferroelectric ordering already at room temperature. In these samples, the usable strain is relatively stable over a...

Preparation, structures, and multiferroic properties of single phase Bi1−xLaxFeO3(x=0–0.40) ceramics

A simple and effective method that solid state reaction followed by quenching process is developed to prepare multiferroic La-substituted BiFeO3 (Bi1−xLaxFeO3 (BLFOx) with x=0–0.40) ceramics. X-ray diffraction, x-ray photoelectron spectroscopy, and inductively coupled plasma studies show that the ceramics prepared under the optimized conditions are single phase. A phase transition from rhombohedral to orthorhombic phase is observed near x=0.30. This transition has great effects on the multiferroic properties. Magnetic and electric measurements reveal that the BLFO0.30 has enhanced multiferroic properties with two times remnant magnetization and polarization of 0.041emu∕g and 22.4μC∕cm2, respectively. The enhanced multiferroic properties are attributed to the enhanced magnetoelectric interaction, which results from the La substitution-induced destruction of the spin cycloid. These results show that BiFeO3-based perovskite solid solution with no other ferroelectric end member can have improved multiferroic ...

Raman spectroscopy of (Mn, Co)-codoped ZnO films

Raman spectra of (Mn, Co)-codoped ZnO films were investigated as functions of laser line and temperature. It is shown that the Raman shifts for different phonon modes exhibit redshift with temperature increasing, which can be attributed to the anharmonic effect in the material. Strong resonant Raman spectra of multi-LO phonons were observed as well for the films with the exciting photon energy higher than the direct band gap. Compared with the Raman spectra for ZnO films, the Mn, Co codoping effect on the spectra is revealed by the presence of additional phonon modes at 275 and 642cm−1 and another intensive phonon mode at around 524cm−1. With postannealing, Raman intensity of these phonon modes decrease, owing to the incorporation of oxygen into the films. Origins of the different phonon modes in the spectra are discussed as well.

Semiconductor/relaxor 0–3 type composites without thermal depolarization in Bi0.5Na0.5TiO3-based lead-free piezoceramics

Commercial lead-based piezoelectric materials raised worldwide environmental concerns in the past decade. Bi₀.₅Na₀.₅TiO₃-based solid solution is among the most promising lead-free piezoelectric candidates; however, depolarization of these solid solutions is a longstanding obstacle for their practical applications. Here we use a strategy to defer the thermal depolarization, even render depolarization-free Bi₀.₅Na₀.₅TiO₃-based 0-3-type composites. This is achieved by introducing semiconducting ZnO particles into the relaxor ferroelectric 0.94Bi₀.₅Na₀.₅TiO₃-0.06BaTiO₃ matrix. The depolarization temperature increases with increasing ZnO concentration until depolarization disappears at 30 mol% ZnO. The semiconducting nature of ZnO provides charges to partially compensate the ferroelectric depolarization field. These results not only pave the way for applications of Bi₀.₅Na₀.₅TiO₃-based piezoceramics, but also have great impact on the understanding of the mechanism of depolarization so as to provide a new design to optimize the performance of lead-free piezoelectrics.

Enhanced electrical properties of c-axis epitaxial Nd-substituted Bi4Ti3O12 thin films

High-quality c-axis epitaxial ferroelectric thin films of Bi4Ti3O12 (BTO) and Nd-substituted BTO, Bi3.15Nd0.85Ti3O12 (BNT), were prepared on (001)-LaNiO3-coated (001) LaAlO3 substrates by pulsed-laser deposition. The epitaxial alignments were established by the x-ray diffraction, including θ–2θ and φ scans. Compared to the BTO films, the BNT films have significantly improved electrical properties with about 2 times larger remanent polarization, 0.6 times lower coercive field, better fatigue-resisting characteristics, and 1.7 times larger dielectric constant. These results showed experimentally that Nd substitution could enhance the c-axis electrical properties of BTO. The reason for the improved properties of BNT films was discussed.

Structure, optical, and magnetic properties of sputtered manganese and nitrogen-codoped ZnO films

To realize the hole-mediated ferromagnetism, manganese and nitrogen-codoped ZnO (Zn1−xMnxO:N) films were prepared on sapphire (0001) by reactive radio-frequency (rf) magnetron sputtering from Zn0.97Mn0.03O ceramic targets using N2 gas. X-ray photon spectra reveal that the doped Mn ions are mainly in divalent states and the coexistence of O–Zn and N–Zn bonds in the films. According to the absorption spectra, the band gap of Zn0.97Mn0.03O:N films is about 3.15eV, which is slightly lower than that of ZnO films (3.20eV). Compared with Zn0.97Mn0.03O films, ferromagnetic behavior of Zn0.97Mn0.03O:N films were significantly changed with a coercivity of about 70Oe, a saturation magnetization of 0.92μB∕Mn2+ and a remanance over 0.15μB∕Mn2+ at 300K, while at 10K, they increased to be about 110Oe, 1.05μB∕Mn2+ and 0.23μB∕Mn2+, respectively. However, rapid thermal annealing treatment in pure oxygen results in a significant decrease on the magnetic properties of the films.